WASHINGTON, DC - SEPTEMBER 09: Passengers wait to board trains at Union Station September 9, 2011 in Washington, DC. U.S. government officials said yesterday evening that there is a "specific, credible but unconfirmed threat" to the U.S. homeland as the 9/11 anniversary approaches and that New York and Washington were specifically mentioned as possible targets. (Photo by Chip Somodevilla/Getty Images)

WASHINGTON, DC - SEPTEMBER 09: Passengers wait to board trains at Union Station September 9, 2011 in Washington, DC. U.S. government officials said yesterday evening that there is a "specific, credible but unconfirmed threat" to the U.S. homeland as the 9/11 anniversary approaches and that New York and Washington were specifically mentioned as possible targets. (Photo by Chip Somodevilla/Getty Images)

WASHINGTON, DC - SEPTEMBER 09: U.S. Capitol Police photograph a group of tourists from Singapore who were stopped during a search of a taxi cab near the U.S. Capitol on September 9, 2011 in Washington, DC. Random searches of vehicles in the nation's capital are underway following a report of a "specific, credible, but unconfirmed" threat of an attack in New York City or Washington using a car or truck bomb to coincide with the 10th anniversary of the September 11 attacks. (Photo by Win McNamee/Getty Images)

NEW YORK, NY - SEPTEMBER 09: Police keep watch at a check point near the World Trade Center site on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: Police keep watch at a check point near the World Trade Center site on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: Police keep watch at a check point near the World Trade Center site on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: People ride the New York City subway into Manhattan during the morning commute on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: A man photographs Manhattan from the Brooklyn Bridge on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: Under a dark cloud, a helicopter hovers over the World Trade Center site in lower Manhattan on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: A New York City subway enters a station while en route into Manhattan during the morning commute on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: People ride the New York City subway into Manhattan during the morning commute on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)

NEW YORK, NY - SEPTEMBER 09: Police keep watch at a check point near the World Trade Center site on September 9, 2011 in New York City. Security is being tightened in New York and Washington, DC following the detection of a possible terror threat tied to the tenth anniversary of the 9/11 terror attacks. New York City and the nation are preparing for the 10th anniversary of the terrorist attacks on lower Manhattan, which resulted in the deaths of 2,753 people at the World Trade Center. (Photo by Spencer Platt/Getty Images)